The present invention relates to a device for lubricating a step-up/down mechanical assembly, particularly a transmission gearbox, especially the main transmission gearbox of a helicopter.
It is known that in step-up/down mechanical assemblies of this type, the oil or lubricating fluid fulfills two functions: on the one hand, it reduces friction at gears, bearings and joints and, on the other hand, it takes away power losses in the form of heat.
The first function, that of reducing the coefficients of friction, does not require a high oil flow rate, the essential thing being that a film of oil be maintained between the various contacting parts. By contrast, its importance to the correct operation of the mechanical assembly is fundamental; in the event of failure, the increase in the coefficients of friction leads to an increase in the heat energy, which may soon manifest itself as surface damage to the contacting parts.
The second function of the oil, which consists in transporting heat energy from the areas in which it is generated (gears, bearings, joints, etc.) to dissipation zones (radiators, walls of casings, etc.), for its part requires a higher oil flow rate.
As far as this second function in particular is concerned, devices arranged locally inside the step-up/down assembly and which are capable of fulfilling an acceptable backup role by causing local recirculation of a reserve of oil between hot and confined zones and cooler zones, such as the walls of casings, are known. In mechanical assemblies which transmit relatively low powers, these devices may even be enough to completely fulfill this second function.
The major benefit of backup devices is, however, that they provide an answer to the problem posed by the loss of lubrication of the moving parts of the step-up/down mechanical assembly in question
Specifically, it is known that in the event of a loss of oil from the lubrication device, for example as a result of a leak in the lubrication circuit or from a seal, the increase in the coefficients of friction and the fact that the thermal energy is practically no longer removed, means that the temperatures of the various parts of the step-up/down assembly increase considerably. This in particular results in damage to the surfaces and degradations in the quality of the materials, which may lead to the destruction of parts such as bearings or teeth.
When said step-up/down assembly is a helicopter transmission gearbox, particularly the main transmission gearbox, the aforementioned consequences entail that, for safety reasons, when oil is lost from said transmission gearbox, the helicopter be set down quickly. This constraint may have very serious consequences when overflying certain zones, such as maritime zones, forests, urbanized zones or enemy territory (in the case of military applications), in which landing or ditching is difficult, or even impossible.
It will be noted that the survivability or operability of the helicopter is lower, the heavier the helicopter, because the powers transmitted by the transmission gearboxes increase out of proportion with their size. In particular, as far as most existing heavyweight helicopters are concerned, the operability following a total loss of lubrication is very low, or even non-existent.
Various backup or survival devices for transmission gearboxes, for the event of the malfunctioning of the transmission-gearbox lubricating system, are known.
In particular, the document FR-2 685 758 discloses a backup lubricating and cooling system for helicopter transmission gearboxes. This lubricating system comes into operation in the event of total failure of the existing lubricating circuit or circuits. For this purpose, this known system has an additional lubricating circuit which is independent of the existing circuit or circuits and which in particular comprises a reservoir, a supply line, pressurizing means or a pump, and a distribution circuit. This known system is therefore bulky, heavy and expensive.
Another source, document FR-1 094 563, discloses a lubricating device for a ball bearing. In this case, the lubricating fluid used for lubrication is accumulated in a rotary casing, which it leaves via a number of stationary tubes which are immersed via an open and elbowed end, in a layer of rotating oil centrifuged out to the periphery of the casing. The lubricating oil flows under the action of the centrifuging pressure.
However, the oil thus collected is simply returned to a specific oil reservoir of the lubricating device.
Furthermore, document FR-2 479 946 discloses a lubricating oil-return circuit for coupling mechanisms. For this purpose, the casing of such a mechanism is rotary and leads to an accumulation of oil in the form of an annular layer, and a collecting tube, the end of which is elbowed, is immersed in this annular layer to collect the oil.
This return circuit needs two pumps, namely a first pump which is mounted in series with the collecting tube, for conveying the fluid to a reservoir, and a second pump for removing the fluid and sending it to the elements that are to be lubricated.
The object of the present invention is to overcome these drawbacks. This invention relates to an effective and not-too-bulky device for lubricating a step-up/down mechanical assembly, which allows said mechanical assembly to continue to operate at least for a given length of time in the event of a leak of lubricating fluid and which, in the case of a helicopter transmission gearbox, allows said helicopter to continue to fly with a transmission of sufficient power for the longest possible determined length of time.
To this end, according to the invention, said device for lubricating a step-up/down mechanical assembly, particularly a transmission gearbox, said lubricating device comprising:
a reservoir for collecting lubricating fluid from said step-up/down mechanical assembly;
a circuit for distributing lubricating fluid;
at least one scoop, a first end of which enters said collection reservoir; and
means capable of creating a relative movement between at least said first end of the scoop and said collection reservoir, which causes lubricating fluid to be made to enter said scoop via said first end, is noteworthy in that said means create a relative movement which causes, in the lubricating fluid in said collection reservoir, a dynamic effect such that it causes said lubricating fluid to flow through said distribution circuit in such a way as to convey it to at least one element of said step-up/down mechanical assembly that is to be lubricated.
Thus, by virtue of the invention, lubrication is achieved by re-using the oil or lubricating fluid collected (by trickling) in said collection reservoir. In consequence, in the event of a leak or loss of some of the oil, lubrication can be continued using the collected remaining proportion of the oil, which allows said step-up/down mechanical assembly to continue to operate for a certain length of time yet after the incident which caused the loss of oil.
In particular, in the case of a helicopter transmission gearbox, this length of time may be such that it allows said helicopter to land in complete safety at a safe place. According to the invention, as will be seen in greater detail hereinbelow, in order to lengthen this period of operation in the event of an oil leak, the lubrication implemented by the device according to the invention performs the exclusive lubrication of one or more vital parts of said step-up/down assembly, such as the gears.
Furthermore, by virtue of the invention, the lubricating fluid is made to circulate in said distribution circuit by the dynamic effect or force produced by the relative movement of the collecting reservoir and said first end of the scoop. In consequence, the device according to the invention requires no pump or other known means for causing this circulation, thus making it possible to obtain a lightweight, non-bulky and inexpensive device.
Furthermore, the trickling of lubricating fluid along the walls of the step-up/down assembly, which walls are generally cooler than the moving parts of said assembly, allows the heat energy produced in this assembly to be removed, at least partially, and thus makes it possible to fulfill the second aforementioned function (the removal of heat) of the lubricating fluid.
It will be noted that unlike the means described in the aforementioned documents FR-1 094 563 and FR-2 479 946, the device according to the invention requires no auxiliary means, such as pumps for example, for performing lubrication, that it to say for conveying the lubricating fluid to the elements that are to be lubricated. This leads to numerous advantages, particularly as regards the cost and space requirement, which are lowered.
Of course, the device according to the invention can be used constantly as a complement, in addition to a customary main lubricating circuit, or simply as a backup device for such a main lubricating circuit in the event of failure of the latter.
However, it is also conceivable for the device according to the invention to be used alone, as the main lubricating circuit, on step-up/down mechanical assemblies whose size and transmitted power do not require high flow rates of lubricating fluid.
Advantageously, said relative movement is a rotary movement about an axis of rotation and said first end of the scoop has an elbow, the free end of which is located in a plane roughly orthogonal to said axis of rotation. Instead of being elbowed, said first end may also be simply inclined.
In a first simplified alternative form, said free end of the elbow is roughly straight, whereas, in a second alternative form, said free end of the elbow is roughly in the shape of an arc of a circle, the center of which roughly corresponds to said axis of rotation, which makes it easier for fluid to enter the scoop and circulate in said distribution circuit.
In a first embodiment, said scoop is fixed to a structure of said step-up/down mechanical assembly and said collection reservoir is mounted so that it can rotate with respect to said structure, and said means for creating a relative movement comprise means for causing said collection reservoir to rotate.
Furthermore, in a second embodiment, said collection reservoir is fixed to a structure of said step-up/down mechanical assembly, at least said first end of the scoop is mounted in such a way that it can rotate with respect to said structure, and said means for creating a relative movement comprise means for causing at least said first end of the scoop to rotate.
The aforementioned rotary movement, regardless of the embodiment considered, subjects the lubricating fluid from the collection reservoir to, on the one hand, a rotary movement and, on the other hand, a centrifugal force which together generate the dynamic effect that causes some of said lubricating fluid to circulate into said scoop. This dynamic effect makes it possible to overcome the pressure drops in the distribution circuit and ensure a sufficient flow rate of lubricating fluid.
Furthermore and advantageously, the device according to the invention comprises means, for example fins or cavities, located in said reservoir and capable of reducing the turbulence of said lubricating fluid, which turbulence is generated by said relative movement, which makes it possible to encourage the lubricating fluid to circulate in said scoop and also provides for better effectiveness of the device according to the invention.
A second end of the scoop (or other ends of the scoop), said scoop preferably being produced in the form of a metal pipe, which is the opposite end to said first end opening into the reservoir, may be produced and arranged in various ways. In particular:
it may be arranged directly facing the element that is to be lubricated, for example being equipped with a nozzle for this purpose. Spraying may then be obtained simply by virtue of the dynamic effect (or force) caused on the lubricating fluid by the aforementioned relative movement; and/or
it may open into an auxiliary reservoir intended to receive the lubricating fluid.
In a first alternative form, said auxiliary reservoir is situated in such a way as to cause said lubricating fluid to reach said element that is to be lubricated under the effect of gravity.
Thus, some of the collected, lubricating fluid can be kept in said auxiliary reservoir, particularly for a later use, for example in the event of a loss of lubricating fluid.
In a second alternative form, said auxiliary reservoir is rotated in such a way as to cause the lubricating fluid to reach the element that is to be lubricated through a centrifugal effect.
The present invention also relates to a system for lubricating a step-up/down mechanical assembly, particularly a transmission gearbox, said lubricating system comprising a customary main lubricating circuit.
According to the invention, this lubricating system is noteworthy in that it additionally comprises a lubricating device such as the aforementioned one and in that this lubricating device is arranged in such a way as to lubricate exclusively at least one vital part (gear, bearing, etc.) of said step-up/down mechanical assembly.
In the context of the present invention, the expression xe2x80x9cvital partxe2x80x9d is to be understood as meaning a part of the step-up/down assembly which has to be lubricated in order to be able to continue to fulfill its functions or, more specifically, the unlubricated running time of which is very short. As a preference, the lubricating device is intended to lubricate a certain number of parts of said step-up/down assembly, the respective unlubricated running times of which are the shortest.
Thus, by virtue of the addition of said lubricating device, the lubricating system according to the invention can continue to lubricate the vital part or parts of the step-up/down mechanical assembly in the event of the failure of the main lubricating circuit, which makes it possible to lengthen the running time of said mechanical assembly in such a situation.